Transcript Overview and Motivation

Low Emittance Tuning in CESR TA
D. Rubin
Cornell University
April 18, 2009
Objectives
Attain sufficiently low vertical emittance to enable exploration of
- dependence of electron cloud on emittance
- emittance diluting effect of e-cloud
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Design/deploy low emittance optics (1.5 < Ebeam< 5.0 GeV)
– Exploit damping wigglers to reduce damping time and emittance
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Establish efficient injection of electrons and positrons
Develop beam based techniques for characterizing beam position monitors
– BPM offsets, Gain mapping, ORM and transverse coupling measurements > BPM tilt
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And for measuring and minimizing sources of vertical emittance including
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Misalignments
Orbit errors
Focusing errors
Transverse coupling
Vertical dispersion
Develop single bunch/single pass measurement of vertical beam size
Characterize current dependence of lifetime in terms of beam size
Measure dependencies of beam size/lifetime on
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Beam energy
Bunch current
Species
Etc.
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Low Emittance Optics - 2GeV
Twelve 1.9T wigglers in zero dispersion straights yield 10-fold reduction
in radiation damping time and 5-fold reduction in horizontal emittance
- Conditions are well established
- Injection capture efficiency for both electrons and positrons is good
- Low current (<1mA/bunch) lifetime ~ hours for both species
wigglers
Energy [GeV]
Wiggler[T]
Qx
Qy
Qz [4.5MV]
x[nm]
p
l[mm]
E/E[%]
April 18, 2009
2.085
1.9
14.57
9.6
0.055
2.6
6.76e-3
12.2
0.81
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Low Emittance Optics - 5GeV
Six 1.9T wigglers in L0 - zero dispersion
(Arc vacuum chambers cannot tolerate wiggler radiation)
wigglers
(Wigglers off)
Energy [GeV] 5.0
Wiggler[T]
1.9
Qx
14.57
Qy
9.6
Qz [8 MV]
0.043
x[nm]
35
p
6.23e-3
l[mm]
15.6
rad [ms]
20
E/E [%]
0.93
April 18, 2009
0
60
9.4
30
0.58
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Measure -phase and coupling
Low emittance tuning
Experimental procedure
LET - initialization
-Measure and correct orbit using
all dipole correctors
-Measure -phase and
transverse coupling
(Phase measurement insensitive
to BPM offset, gain, and calibration errors)
Measurement at January 09 startup
after 2 month CHESS (5.3GeV) run
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Low emittance tuning
Experimental procedure
LET - initialization
-Measure and correct orbit using
all dipole correctors
- Correct -phase using all 100
Remeasure - (
)
-Correct transverse coupling using 14
skew quads. Remeasure (
)
-phase and coupling after correction
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Low emittance tuning
Orbit
A feature of the orbit is
the closed horizontal bump
required to direct xrays
onto x-ray beam size monitor
xBSM bump
Ability to correct the vertical
orbit is limited by BPM
resolution and irreproducibility
-Measure and correct vertical
dispersion
using skew quads (14) and
vertical steering (100)
Residual vertical dispersion
RMS ~ 2.4cm - Signal or noise ?
Difficulty modeling suggests that
it is noise.
Accuracy of dispersion
measurement is limited by BPM
Note: Residual vertical dispersion 1cm, corresponds to v ~ 10pm
systematics
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Magnet Alignment
Survey network complete
- Quad offset  ~ 134m
- Bend roll  ~ 160rad
- Sextupoles ?
Fixed with respect to adjacent quadrupole
Investigating systematic ~ 350 m offset
Designing fixtures for correction
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BPM characterization
BPM tilt
- “measured” v ~ h
where  = BPM tilt
Since <h> ~ 1m, BPM tilt
must be less than 10mrad
if we are to achieve v <1cm
We use ORM and phase/coupling
measurement to determine .
ORM data set ~ 140 measured orbit differences
- Take data set 1
Correlation of fitted BPM tilt ()
- Vary 8 skew quads and repeat
 < 10 mrad
- Take data set 2
Fit each data set using all quad(k), skew(k), BPM()
Consistent with BPM(x) ~35m
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BPM upgrade
Existing BPM electronics measure
stretched signal and share common signal
processing via mechanical relays
New system
- Bunch by bunch/turn by turn
digitization
- 4ns bunch spacing
- (x) < 10m
With the new system we will
measure:
- Quad - BPM offset < 50 m via beam based
alignment (Vary quad K to find center)
-  ~ x/(E/E) ~ 10m /10-3 ~ 1cm
- Clean measurement of C11, C12, C22
discriminates BPM tilt and transverse
coupling (C12 independent of tilt)
Status
Infrastructure (cables,crates, etc.) fully
deployed in tunnel
Conversion from old system to new is
underway - taking care to maintain full
functionality during the transition
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Lifetime
Touschek lifetime
CesrTA operates in a regime where lifetime is current dependent
Intrabeam scattering kicks particles outside of energy aperture
Touschek lifetime depends on energy aperture
The Touschek parameter (b) decreases with:
- increasing beam size
(introducing v in damping wigglers)
- increasing bunch length
(reduced accelerating voltage)
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Lifetime
Touschek lifetime (and Touschek parameter [b] ) depends on
- dynamic energy acceptance
- RF accelerating voltage
- vertical emittance
The curves in the plot show theoretical
dependence of Touschek parameter on
accelerating voltage for different combinations
of dynamic acceptance and vertical emittance
The data (filled circles) are consistent with
0.72% energy acceptance and
32pm vertical emittance
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Dynamic energy aperture
Interpretation of lifetime measurements requires knowledge of dynamic energy acceptance
Tracking study indicates energy acceptance ~1.8%
(lifetime measurements suggest significantly smaller energy acceptance)
Tracking model includes:
-magnet misalignments
-wiggler and quadrupole nonlinearity
-Orbit errors
Energy acceptance ~ 1.8%
Nonlinearity of dipole
correctors and sextupoles
has not yet been included.
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Energy Acceptance
Determine energy acceptance experimentally
by measuring lifetime vs energy offset
E/E~1/p(fRF/fRF)
Energy acceptance ~ 0.8%
This direct measurement of
energy acceptance is consistent
with lifetime measurements and
v ~ 32pm
It remains for us to reconcile
measurement and tracking
calculation of energy acceptance.
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Xray Beam Size Monitor
diamond window
turbo
gate valve
x rays
Detector Box
Fast gate
valve
low quality
vacuum (10-3)
Rough
At 2GeV beam energy, xrays from a
bending magnet are at such low energy
that a Be window is not sufficiently transparent
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high quality
vacuum (10-7)
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x rays
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xBSM Snapshots
• Scan of coupling knob
• Coded aperture measurements
• Smallest recorded size:
~15 m v~ 37pm (preliminary)
Fresnel Zone Plate
Monochromatic beam
Simulations
Coded
Aperture
First measurement
<45 m
beam size
White beam
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Coded Aperture
CESR condition same
as for 18um FZP
All measurements with single diode
Diode array will provide “real” time
beam size measurement
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Xray Beam Size Measurement
Measure beam size
vs coupling/dispersion knobs
Linear combinations of skew quad correctors
yield closed dispersion/coupling bumps in
the damping wigglers and are used to
tune vertical emittance
Betasing 1/2 - L0 wigglers
Betasing 3/4 - East arc wigglers
Betasing 5/6 - West arc wigglers
Beam size is measured with
the Xray beam size monitor
Knob setting = 0 corresponds to
conditions after low emittance tuning
procedure.
The spread (37pm < v < 60pm) in minimum beam size is
presumably due to knob hysterisis
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~minimum
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Xray beam size Measurement
Consider the dependence of beam
size on Betasing 1 (the knob that
effects , in the L0 wigglers)
Model dependence of vertical emittance on
Betasing 1 is indicated by the black circles in
the plot.
We assume Betasing 1 = 0 corresponds to
zero vertical emittance. (The model
machine)
( ~ v2)
The measured beam size is indicated by the
triangles
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Xray beam size Measurement
If we assume a residual v ~ 1.7cm
then v(0)~ 35pm
(we measure residual v ~ 2cm)
Again, according to the model
calculation, dependence of v on
Betasing 1 is black circles
Model and measurement are
in reasonable agreement
Conclusion from lifetime and
Xbsm measurements is that
v~ 35pm
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Low emittance tuning
- limited by finite v
- Identification of the source
requires better measurement
Consider the effect of sextupole misalignment
The measurement
1. Correct (flatten) orbit
2. Correct coupling with skew quads
3. Measure -phase and coupling
4. Turn off all sextupoles
and re-Measure -phase and coupling
The RMS phase difference is ~ 1°
The RMS coupling difference is~ 4.2%
The measured coupling corresponds
to systematic sextupole vertical
offset of ~1mm
Direct measurement suggest some such offset!
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Low Emittance Tuning
Analysis tools
CESRV is the code that provides
- access to control system to make measurements of
orbit, -phase, transverse coupling, dispersion
- analysis of measurements
(wave analysis, fitting [model to measurement], calibration, etc.)
- access to the control system to load corrections to
steerings, quadrupoles, skew quads, sextupoles …
- data manipulation - plotting, comparison, bookkeeping, etc.
CESRV runs on linux (as well as VMS)
- Linux / control system communication is transparent to user
> Real time measurement/analysis/correction
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LET Status
- Survey and alignment
Quadrupole offsets and rolls, and bend rolls within tolerances
- Quadrupole focusing errors corrected
- Coupling corrected < 1%
- Vertical dispersion ~ 2cm (the goal is 1cm)
- Measured vertical emittance (lifetime and XBSM) ~ 35pm
( corresponds to v(RMS)~ 1.8cm)
 Residual vertical dispersion dominates vertical emittance
- Our ability to correct vertical dispersion limited by BPM resolution
-Implementation of digital BPM electronics (May-June 09 run) will provide required
resolution/reproducibility
[Candidate source of dispersion is sextupole misalignment
(Developing a plan for measuring and correcting offset errors)]
- Analysis software and infrastructure is flexible, well tested, and mature
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Acknowledgements
J. Alexander, M. Billing, J. Calvey, S. Chapman, G. Codner, J. Crittenden, J. Dobbins, G. Dugan, M. Forster, R.
Gallagher, S. Gray, S. Greenwald, D. Hartill, W. Hopkins, J. Kandaswamy, D. Kreinick, Y. Li, X. Liu, J. Livezey,
V. Medjidzade, R. Meller, S. Peck, D. Peterson, M.Rendina, D. Rice, N. Rider, D. Sagan, J. Sexton, J. Shanks, J.
Sikora, K. Smolenski, C. Strohman, A. Temnykh, M. Tigner, W. Whitney, H. Williams, S. Vishniakou, T. Wilksen
(CLASSE, Cornell University)
K. Harkay (Argonne National Lab)
R. Holtzapple (California Polytechnic Institute)
E. Smith (CCMR, Cornell University)
C. Connolly, E. Fontes, A. Lyndaker, P. Revesz, J. Savino, R. Seeley (CHESS, Cornell University)
J.Jones, A. Wolski (Cockcroft Institute)
Y.He, M. Ross, C.Y.Tan, R. Zwaska (Fermi National Accelerator Laboratory)
J.Flanagan, P. jain, K. Kanazawa, K. Ohmi, Y. Suetsugu (KEK Accelerator Laboratory)
J. Byrd, C.M.Celata, J. Corlett, S. De Santis, M. Furman, A. Jackson, R. Kraft, D. Munson, G. Penn, D. Plate, A.
Rawlins, M. Venturini, M. Zisman (Lawrence Berkeley National Laboratory)
D. Kharakh, M. Pivi, L. Wang (SLAC National Accelerator Laboratory)
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AC dispersion measurement
v(AC)
Achieving emittance target
depends on reducing vertical
dispersion to < 1cm. Presently
limited by marginal quality
of measurement
AC technique may give
Requisite resolution but
not yet
v(DC)
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